Our only surviving reference to the Goddess Sinthgunt comes from the Old High German Merseburg Incantation (also known as the “Horse Cure Charm”), which dates to around the 9th or 10th Century.
When it comes to religious, pagan celebrations most people are familiar with the eight holy days or sabbats that comprise the Wheel of the Year, such as Lugnasadh. In the Northern Tradition, we do not call these celebrations sabbats. Instead, based on words (like the Old Norse hátíðir) used to describe the most holy of these celebrations (like Yule) as high tides, we tend to call the various religious celebrations we recognize today as holy tides (since not all of the holy tides are considered high tides).
Since we practitioners of the Northern Tradition are dealing with a general umbrella culture that existed in vast plurality we look to ancient Germanic, Scandinavian (Norse, Icelandic, Sweden) and Anglo-Saxon sources. It is important to understand that these ancient cultures reckoned time in different ways in comparison to one another or to the modern world. They existed in different latitudes, lived amongst different types of geography with unique climate conditions that affected the local agricultural cycle. This means that sometimes the timing between when one group would celebrate and another would celebrate a similar type of holy tide could be several weeks apart.
Sometimes we can see an obvious and clear link between these cousin cultures to a specific holy tide like Yule, in other cases things are a bit less clear, or the celebrations of the different groups can sometimes seem vastly different even when they have a similar root. Case in point: the Northern Tradition holy tides in August known as Hlæfmæsse, or Freyfaxi.